Subterranean wells such as those used for the production of oil and gas commonly use tubular products called "casing" to line the interior walls of the well. The casing in most wells is formed of metallic materials and, most commonly, steel. The casing is in contact directly or indirectly with the surrounding earth. As a consequence, the casing is subjected to the possibility of the passage of electrical currents away from it and, therefore, corrosion.
It is possible to protect the well casing by imposing an artificial current of equal or greater magnitude and in the opposite direction to the natural current leaving the casing. To do so properly and economically, it is necessary to accurately determine the magnitude of the current picked up by the well casing. This can be accomplished by measuring the voltage (IR drop) at various locations in the well and dividing this value by the resistance of the casing at that location. Estimates of the resistance of the casing can be obtained from the casing manufacturer. However, gross inaccuracies can arise from use of these data. Errors can arise because manufacturing variations in the casing wall thickness can range up to .+-.12%. Further errors can arise because the casing has become corroded, or because of material variations within the casing.
Various devices have been described which are intended to measure casing resistance. For example, U.S. Pat. No. 4,431,963 (Walkow, which is incorporated herein by reference for all purposes) describes such a device. Four contacts (C1, C2, C3, and C4) are raised and lowered in a wellbore. The natural casing potential between the innermost contacts (C2 and C3) is determined at various locations in the wellbore. A known current is then passed between the outer contacts (C1 and C4) while the potential drop between the innermost contacts (C2 and C3) is measured. From these values, the casing resistance at any point in the casing, the natural current flow from the casing and, ultimately, the metal loss rate in the casing can be determined. Variations in this method are also described using AC, and pulsed current flows.
There are several inherent disadvantages of the above system (and those of a similar nature). Initially, it has been found by the present inventors that the rollers used as contacts (C1, C2, C3, and C4) in Walkow can become fouled with paraffin, pipe dope, corrosion materials, and the like. Therefore, inadequate contact between the tool and the casing wall is made. This can result in inaccurate casing resistance values. Further, it has been found that the low current imposed on the casing (for the purpose of determining resistance) can result in significant errors because the "noise" in the measured values is almost as large as the values themselves. The device described by Walkow cannot impose large currents on the casing because it is limited by the capabilities of a typical logging cable to transmit such current from the surface. Further errors can arise due to inadequacies in the method and apparatus used to transmit data to the surface.
It is desirable, therefore, to devise a tool which maintains acceptable contact with the well casing and which can impose significant current on the casing for the purpose of determining casing resistance. It is further desirable to devise a method and apparatus which can transmit data to the surface without data loss or distortion.